Automatic transmission fluids (ATF) are non-compressible lubricant compositions containing a number of conventional additives. As typically used, an ATF serves as a hydraulic fluid, activating and engaging gears in the transmission by a series of valves and other hydraulic circuits, and as a lubricant for the hydraulic pump used to provide hydraulic pressure for operation of the transmission.
ATFs generally contain detergent and similar additives that tend to produce foam if air is entrained into the fluid. Additionally, impurities are produced in the fluid over time (for example, by oxidation or degradation of the base oil), some of which may contribute to a foaming tendency in the ATF. Entrained air in an ATF is a problem because the air alternately expands in the low pressure inlet side of pump, and quickly contracts or is compressed as the fluid passes through the pump to the high pressure outlet side.
The resulting implosion of air bubbles on the outlet side causes pressure ripples in the hydraulic pump. The pressure ripples can cause objectionable audible noise, manifested as “pump whine” in some transmissions. New automatic transmissions, such as continuously variable transmissions (CVT), with their compact sumps and high pump pressures, have raised the possibility of consumer reaction to the noise. In response, a number of OEMs have taken steps to reduce the air level in the fluid of their new transmissions by isolating or baffling the internal rotating components to separate them from the fluid, or by introducing aeration additives into the ATF to help the oil release the entrained air more quickly or otherwise reduce the level of entrained air. Additionally, conventional antifoam agents have been employed to help dissipate surface air bubbles.
Antifoams work in part by being insoluble in ATF. As such, they function in part by having a preferential tendency to reside on the surface of bubbles. However, in hydraulic pumps, the act of adiabatically compressing entrained air on the outlet side causes the surface of air bubbles to reach high temperatures. In some cases, the temperature may reach 500° C. or greater. At such elevated temperatures, the antifoam agent is subject to thermal degradation. Because some conventional antifoam agents, such as polydimethylsilicone, are thermally stable only up to about 200° C., they are subject to thermal breakdown in the modern transmission environment.
Antifoam agents need to be dispersed, but not dissolved, in the form of liquid droplets above a minimum size in order to be functional in an ATF. Thermal degradation of the molecules of the antifoam agent inhibits the ability of the antifoam agent molecules to form droplets of effective size. Thermal degradation of the antifoam agent and/or change in the properties of the ATF can result in antifoam agent molecules that are undesirably further solubilized (i.e., dissolved) in the ATF, such that they are no longer functional as antifoams; they may even become foaming agents.
The insolubility of the antifoam agents leads to some difficulties that must be addressed by the formulator of ATF. Typically, the antifoam agent is denser than the base fluids and tends to fall out during shipping and storage before being added to the transmission. In practice, this limits the amount of antifoam agent that can be incorporated or dispersed into the ATF by the fluid supplier. Alternatively, a formulated ATF may be re-dispersed prior to filling the transmission, but the extra step creates additional expense in the manufacturing process.